D3D11: Apply a non-null blendstate in masked depth clear. In https://chromium-review.googlesource.com/c/453888/, we changed how blendstates were applied in masked clears. This change would apply a null blendstate when all color channels were disabled, but this seems to have an issue on Intel Cherry View drivers. Work around this issue by restoring the prior functionality of making a simple masked blend state. Also clean up some of the code style in the Clear11 class. BUG=chromium:730126 Change-Id: I9a4044201b2f07e9483525513a59e19bb2a8bcd3 Reviewed-on: https://chromium-review.googlesource.com/533684 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>